Apparatus and method for monitoring a thickness of a silicon wafer with a highly doped layer

ABSTRACT

Apparatus for monitoring a thickness of a silicon wafer with a highly-doped layer at least at a backside of the silicon wafer is provided. The apparatus has a source configured to emit coherent light of multiple, wavelengths. Moreover, the apparatus comprises a measuring head configured to be contactlessly positioned adjacent the silicon wafer and configured to illuminate at least a portion of the silicon wafer with the coherent light and to receive at least a portion of radiation reflected by the silicon wafer. Additionally, the apparatus comprises a spectrometer, a beam splitter and an evaluation device. The evaluation device is configured to determine a thickness of the silicon wafer by analyzing the radiation reflected by the silicon wafer by an optical coherence tomography process. The coherent light is emitted multiple wavelengths in a bandwidth b around a central wavelength w c .

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of a GreatBritain patent application, serial number GB 1004116.8, filed on Mar.12, 2010, the disclosure of which is incorporated herein by referencefor all purposes.

TECHNICAL FIELD

The invention relates in general to an apparatus and a method formonitoring a thickness of a silicon wafer, in particular a silicon waferwith a highly-doped layer at least at a backside of the silicon wafer,and an apparatus for thinning a silicon wafer.

BACKGROUND INFORMATION

A method and apparatus for controlling the thickness of a semiconductorwafer during a backside grinding process are disclosed in U.S. Pat. No.6,368,881B1. Optical measurements of the wafer thickness during abackside grinding process are used to determine the endpoint of thegrinding process. Furthermore, the method may be used to determine ifwedging of the semiconductor occurs and, if wedging does occur, toprovide leveling information to the thinning apparatus such that agrinding surface can be adjusted to reduce or eliminate wedging.

However, it is believed that this method and apparatus are not suitableto reliably measure wafer thicknesses for silicon wafers with ahighly-doped silicon layer on the backside of the silicon wafer due to alow-contrast interference signal.

It is therefore desirable to provide apparatus and a method formonitoring a thickness of a silicon wafer with a highly-doped layer atleast at a backside of the silicon wafer, which provide a sufficientlyhigh-contrast interference signal.

SUMMARY

In response to these and other problems, in one embodiment, there isdisclosed an apparatus for monitoring a thickness of a silicon waferwith a highly-doped silicon layer at least at a backside of the siliconwafer.

In other embodiments, there is disclosed an apparatus for thinning asilicon wafer and to advantageously determine the thickness of thesilicon wafer with high accuracy.

In yet other embodiments, there is disclosed a method for monitoring athickness of a silicon wafer with a highly-doped silicon layer on atleast at a backside of the silicon wafer.

These and other features, and advantages, will be more clearlyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings. It is important to note the drawings arenot intended to represent the only aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. is a schematic view illustrating an apparatus for thinning asilicon wafer according to one embodiment of the invention;

FIG. 2 is a flow chart illustrating a method for thinning a siliconwafer according to one embodiment of the invention.

Although the drawings are intended to illustrate one embodiment of thepresent invention, the drawings are not necessarily drawn to scale.

DETAILED DESCRIPTION

Specific examples of components, signals, messages, protocols, andarrangements are described below to simplify the present disclosure.These are, of course, merely examples and are not intended to limit theinvention from that described in the claims. Well known elements arepresented without detailed description in order not to obscure thepresent invention in unnecessary detail. For the most part, detailsunnecessary to obtain a complete understanding of the present inventionhave been omitted inasmuch as such details are within the skills ofpersons of ordinary skill in the relevant art. Details regarding controlcircuitry or mechanisms used to control the rotation of the variouselements described herein are omitted, as such control circuits arewithin the skills of persons of ordinary skill in the relevant art.

Turning now to FIG. 1, there is presented one embodiment apparatus 34for thinning a silicon wafer 1 according to an embodiment of theinvention. The silicon wafer 1 comprises a highly-doped layer (notshown) at least at a backside 30 of the silicon wafer 1.

The apparatus 34 comprises a system for removing at least a portion ofthe backside 30 of the silicon wafer 1 and apparatus for monitoring athickness of the silicon wafer 1, in particular during a backsidegrinding process.

The system for removing comprises a grinding wheel 14 and a plate 15,wherein the plate 15 is configured to retain the silicon wafer 1 forexample by means of a chuck (not shown). The grinding wheel 14 isrotatable about its own rotational axis 36 as schematically illustratedby an arrow A and the plate 15 is rotatable about its own rotationalaxis 37 as schematically illustrated by an arrow B. A motor 16 drivesthe rotatable grinding wheel 14 and a motor 17 drives the rotatableplate 15.

The apparatus for monitoring the thickness of the silicon wafer 1comprises a measuring head 3, which is arranged contactlessly adjacentthe backside 30 of the silicon wafer 1. Moreover, the apparatus formonitoring comprises a light source 2 which is a spectral broadbandlight source or a tunable laser source configured to emit coherent lightat multiple wavelengths around a central wavelength w_(c) in a nearinfrared region in a bandwidth b of several tens of nanometers to about100 nanometers. The light source 2 may be tunable by means of anoscillating micromechanics.

The spectral broadband light source may be selected from the groupconsisting of a light-emitting diode, a semiconductor superluminescentdiode (SLD) and an optically pumped fiber based amplified spontaneousemission (ASE) source, and the tunable laser source may be selected fromthe group consisting of an optically pumped photonic crystal laser and asemiconductor quantum dot tunable laser.

The measuring head 3 is configured to illuminate at least a portion ofthe silicon wafer 1, in particular the backside 30, with the coherentlight of multiple wavelengths of the light source 2 and to receive atleast a portion of radiation reflected by the silicon wafer 1, inparticular by the backside 30 of the silicon wafer 1 and by a front side31 of the silicon wafer 1, wherein the front side 31 is opposite to thebackside 30. This is illustrated schematically by arrows C and D in FIG.1.

The measuring head 3 is arranged in a housing 13 of the apparatus formonitoring. Coherent light of the light source 2 and at least a part ofthe reflected radiation pass through a window 12 of the housing 13,wherein the window 12 is transparent for the light of the light source2. In the illustrated embodiment, the window 12 is transparent forinfrared light.

The housing 13 is preferably waterproof and further comprises aninjection port 33. The injection port 33 is connected to a water tube(not shown) and/or a compressed air tube (not shown). Water and/orcompressed air (not shown) injected into the injection port 33 may runthrough a channel 35, the channel 35 being essentially a part of thespace between the housing 13 and the backside 30 of the silicon wafer 1,and remove grinding debris. This may prevent measurement errors for thethickness measurement caused by the grinding debris. Furthermore, in thecase of water, the water may help to couple light between the window 12and the silicon wafer 1.

The measuring head 3 is coupled to a beam splitter which in theillustrated embodiment is an optical coupler 4 of the apparatus formonitoring by a first optical waveguide 10. The optical coupler 4couples the light source 2 by the first optical waveguide 10 and asecond optical waveguide 9 coupling the optical coupler 4 to the lightsource 2 to the measuring head 3 and coherent light of the light source2 is provided to the measuring head 3 via the second optical waveguide9, the optical coupler 4 and the first optical waveguide 10. Thereflected radiation is provided to a spectrometer 5 of the apparatus formonitoring via the fist optical waveguide 10, and a third opticalwaveguide 11 coupling the optical coupler 4 to the spectrometer 5.Partial intensities I(λ) of the reflected radiation are measured as afunction of the wavelengths λ with an array of photodetectors (notshown) in the spectrometer and a spectrum of the measured partialintensities I(λ) is generated. The spectrum is provided to an evaluationdevice 6 via a signal line 25 which couples the spectrometer 5 to theevaluation device 6.

In the illustrated embodiment, the evaluation device 6 continuouslydetermines the thickness of the silicon wafer 1 using a FD OCT processwhich may be a 1D-se FD OCT process or a 1D-te FD OCT process. Thedetermined thicknesses are then provided via a signal line 23 to acoupling unit 19.

The coupling unit 19 is connected to a monitor 21 by a signal line 29.The determined thicknesses of the silicon wafer 1 may be displayed onthe monitor 21. In the illustrated embodiment, the monitor 21 comprisesa touch screen 22. Via the touch screen 22, a predetermined thicknessfor the thickness of the silicon wafer 1 may be set. The value of thepredetermined thickness is provided via the signal line 29, the couplingunit 19 and a signal line 24 to a control unit 18 for the system forremoving, the signal line 24 coupling the coupling unit 19 to thecontrol unit 18. The control unit 18 is configured to halt the systemfor removing when the determined thickness of the silicon wafer 1reaches the predetermined thickness. This may be achieved by providingan according signal to the motor 16 of the grinding wheel 14 via acontrol line 26 and an according signal to the motor 17 of the plate 15via a control line 27, the control line 26 coupling the motor 16 to thecontrol unit 18 and the control line 27 coupling the motor 17 to thecontrol unit 18.

The control unit 18 comprises a controller 20 configured to adjust therotation speed of the rotatable plate 15 and the rotatable grindingwheel 14 depending on the determined thickness of the silicon wafer 1.This may be achieved by providing according control signals to the motor16 of the rotatable grinding wheel 14 via the control line 26 and to themotor 17 of the rotatable plate 15 via the control line 27.

The apparatus for monitoring further comprises a selection device 7. Adopant concentration of the highly-doped layer of the silicon wafer 1may be input via the touch screen 22 of the monitor 21 and may beprovided to the selection device 7 via a signal line 28 coupling themonitor 21 to the selection device 7. The selection device 7 isconfigured to select the central wavelength w_(c) to be a wavelength forwhich an optical absorption coefficient of the highly-doped layer of thesilicon wafer 1 having the input dopant concentration is a minimum. Inthe illustrated embodiment, the selection device 7 comprises a storagedevice 8 with stored data of a dependence of the absorption coefficienton the dopant concentration. The selection device 7 is configured toselect the central wavelength w_(c) using the data stored in the storagedevice.

In the illustrated embodiment, the highly-doped layer of the siliconwafer 1 comprises silicon with a dopant concentration N, wherein 1*10¹⁹cm⁻³≦N≦1*10²¹ cm⁻³. The highly-doped layer of the silicon wafer 1 may beof n⁺-type conduction or of p⁺-type conduction.

FIG. 2 illustrates a flow chart of a method for thinning a silicon waferaccording to an embodiment of the invention.

In a step 100, a silicon wafer is attached to an apparatus for thinningaccording to one of the embodiments mentioned above. In a step 110, adopant concentration of the highly-doped layer of the silicon wafer isinputted.

In a step 120, a predetermined thickness for the thickness of thesilicon wafer is set and in a step 130, the process of removing at leasta portion of a backside of the silicon wafer is started.

In a step 140, the process of illuminating at least a portion of thesilicon wafer with coherent light of a spectral broadband light sourceor a tunable laser source, receiving at least a portion of radiationreflected by the silicon wafer and determining a thickness of thesilicon wafer by analyzing the radiation reflected by the silicon waferusing a process selected from the group consisting of a 1D-se FD OCTprocess, a 1D-te FD OCT process, a 1D-se TD OCT process and a 1D-te TDOCT process is initiated. The spectral broadband light source or thetunable laser source emits coherent light at multiple wavelengths in abandwidth b around a central wavelength w_(c), wherein a wavelength forwhich an optical absorption coefficient of the highly-doped layer of thesilicon wafer having the input dopant concentration is a minimum lieswithin the bandwidth b.

In a step 150, the removing step is stopped when the determinedthickness of the silicon wafer reaches the predetermined thickness.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many combinations, modifications and variations are possiblein light of the above teaching. For instance, in certain embodiments,each of the above described components and features may be individuallyor sequentially combined with other components or features and still bewithin the scope of the present invention. Undescribed embodiments whichhave interchanged components are still within the scope of the presentinvention. It is intended that the scope of the invention be limited notby this detailed description, but rather by the claims.

For instance, some embodiments may provide for an apparatus formonitoring a thickness of a silicon wafer with a highly-doped siliconlayer at least at a backside of the silicon wafer.

The highly-doped silicon layer at the backside of the silicon wafer mayextend from the outermost surface of the backside into the body of thesilicon wafer or may be arranged in a near surface region of thebackside entirely within the body of the silicon wafer.

In an embodiment, the highly-doped silicon layer may also extendlaterally over the entire backside of the silicon wafer in addition toextending from the outermost surface of the backside into the body ofthe silicon wafer or being arranged in a near surface region of thebackside entirely within the body of the silicon wafer. The high-dopedsilicon layer may form part of a contact for a device structurepositioned within the silicon wafer. For example, the high-doped layermay form part of a drain contact for a vertical transistor such as aMOSFET (Metal Oxide Semiconductor Field Effect Transistor).

The apparatus comprises a light source configured to emit coherent lightat multiple wavelengths. Moreover, the apparatus comprises a measuringhead configured to be contactlessly positioned adjacent the siliconwafer comprising the backside from which at least a portion is to beremoved and configured to illuminate at least a portion of the siliconwafer with the coherent light of multiple wavelengths and configured toreceive at least a portion of radiation reflected by the silicon wafer.The measuring head may be positioned adjacent the backside of thesilicon wafer or adjacent a front side of the silicon wafer beingopposite to the backside. Additionally, the apparatus comprises aspectrometer configured to receive at least a portion of the radiationreflected by the silicon wafer and measure the partial intensities ofthe radiation reflected by the silicon wafer as a function ofwavelength, a beam splitter coupled to the measuring head, the lightsource and the spectrometer for guiding the coherent light of multiplewavelengths to the measuring head and the received radiation to thespectrometer and an evaluation device. The evaluation device isconfigured to determine a thickness of the silicon wafer by analyzingthe radiation reflected by the silicon wafer using an optical coherencetomography process. The light source is configured to emit coherentlight at multiple wavelengths with a bandwidth b around a centralwavelength w_(c), wherein the bandwidth b has limits that are definedsuch that a wavelength for which an optical absorption coefficient ofthe highly-doped layer of the silicon wafer is a minimum lies within thebandwidth b.

The apparatus for monitoring according to the present invention mayprovide light which passed through the silicon wafer and is reflected bya second surface of the silicon wafer with an intensity sufficientlyhigh to produce a high-contrast interference signal with the lightreflected by the first surface of the silicon wafer which is closer tothe measuring head than the second surface by including a wavelength forwhich the optical absorption coefficient of the highly-doped layer ofthe silicon wafer is a minimum within the bandwidth b. The opticalabsorption coefficient for highly-doped silicon increases significantlywith increasing dopant concentration. Including the wavelength for whichthe optical absorption coefficient of the highly-doped layer of thesilicon wafer is a minimum is thus of particular advantage. Theapparatus thus provides a reliable monitoring of the thickness ofsilicon wafers with a highly-doped silicon layer at least at thebackside, in particular for silicon wafers with thicknesses above 50 μm.

In an embodiment, the bandwidth b has limits that are defined such thatthe central wavelength w_(c) is the wavelength for which an opticalabsorption coefficient of the highly-doped layer of the silicon wafer isa minimum. This may enhance the intensity of the light which passedthrough the silicon wafer further, since the central wavelength w_(c) isemitted with the highest intensity within the bandwidth b.

In a further embodiment, the apparatus is an in-situ backside grindingprocess apparatus for monitoring a thickness of the silicon wafer withthe highly-doped silicon layer at least at the backside of the siliconwafer during a backside grinding process and the evaluation device isconfigured to determine the thickness of the silicon wafer by analyzingthe radiation reflected by the silicon wafer at least during thebackside grinding process. This embodiment advantageously provides abackside grinding process monitor, which reliably measures the thicknessof the silicon wafer during a backside grinding process with highaccuracy. Reducing the thickness of silicon wafers is of particularimportance for silicon wafers which are intended to be divided intoplural semiconductor chips with integrated circuits in order to improvelightness and compactness of the semiconductor chips and to reduce theon-state resistance of the semiconductor device, for instance of a powersemiconductor device.

The apparatus may further comprise an input device for inputting adopant concentration of the highly-doped layer of the silicon wafer andmay be configured to select the central wavelength w_(c) to be thewavelength for which the optical absorption coefficient of thehighly-doped layer of the silicon wafer having the inputted dopantconcentration is a minimum.

The apparatus may further comprise a storage device with stored data ofa dependence of the absorption coefficient of silicon on the dopantconcentration and may be configured to select the central wavelengthw_(c) using the data stored in the storage device.

In a further embodiment, the central wavelength w_(c) fulfills therelation 950 nm≦w_(c)≦1150 nm. For said wavelengths, the opticalabsorption coefficient may be minimal for a dopant concentration N ofthe highly-doped layer of the silicon wafer, wherein 1*10¹⁹cm⁻³≦N≦1*10²¹ cm⁻³.

The light source may be a spectral broadband light source or a tunablelaser source.

The spectral broadband light source may be selected from the groupconsisting of a light-emitting diode, a semiconductor superluminescentdiode (SLD) and an optically pumped fiber based amplified spontaneousemission (ASE) source, and the tunable laser source may be selected fromthe group consisting of an optically pumped photonic crystal laser and asemiconductor quantum dot tunable laser.

In a further embodiment, the beam splitter is an optical coupler. Theapparatus for monitoring may further comprise at least one first opticalwaveguide, the at least one first optical waveguide coupling themeasuring head to the optical coupler, at least one second opticalwaveguide, the at least one second optical waveguide coupling theoptical coupler to the light source, and at least one third opticalwaveguide, the at least one third optical waveguide coupling the opticalcoupler to the spectrometer.

The optical coherence tomography process used to determine the thicknessmay be selected from the group consisting of a 1D-se FD OCT process (onedimensional spatially encoded Fourier Domain Optical CoherenceTomography process), a 1D-te FD OCT process (one dimensional timeencoded Fourier Domain Optical Coherence Tomography process).

In an alternative embodiment, the optical coherence tomography processis a 1D-te TD OCT process (one dimensional time encoded Time DomainOptical Coherence Tomography process).

In yet a further alternative embodiment, the optical coherencetomography process is a 1D-se TD OCT process (one dimensional spatiallyencoded Time Domain Optical Coherence Tomography process) and

In a further embodiment which relates to the 1D-se FD OCT process or the1D-te FD OCT process, the spectrometer comprises a diffraction gratingconfigured to expand the spectral distribution of the radiationreflected by the silicon wafer.

In an embodiment, the apparatus for monitoring comprises a housing,wherein at least the measuring head is arranged in the housing andwherein the housing comprises a window, the window being transparent forthe light of the light source. The window is for example an infrared(IR) window. The housing is preferably waterproof.

The invention is further related to an apparatus for thinning a siliconwafer. The apparatus for thinning a silicon wafer comprises a system forremoving at least a portion of a backside of the silicon wafer and theapparatus for monitoring according to one of the embodiments mentionedabove.

An apparatus for thinning according to the invention may advantageouslydetermine the thickness of the silicon wafer with high accuracy.

The system for removing comprises preferably a rotatable grinding wheeland a rotatable plate, the rotatable plate being configured to retainthe silicon wafer.

The thinning apparatus may further comprise a control unit for thesystem for removing and a coupling unit, wherein the coupling unit iscoupled to the evaluation device and to the control unit and wherein thecontrol unit is configured to halt the system for removing when thedetermined thickness of the silicon wafer reaches a predeterminedthickness. This embodiment advantageously provides a thinning apparatuswhich may reliably provide the ground silicon wafer having thepredetermined thickness.

The coupling unit may be a part of the in-situ backside grinding processapparatus or of the control unit.

In a further embodiment, the control unit comprises a controllerconfigured to adjust the rotation speed of the rotatable plate and/orthe rotatable grinding wheel depending on the determined thickness ofthe silicon wafer. This allows for a control of the rotation speed ofthe rotatable plate and/or the rotatable grinding wheel.

The invention further relates to a method for monitoring a thickness ofa silicon wafer with a highly-doped silicon layer at least at a backsideof the silicon wafer, wherein the method comprises the steps ofilluminating at least a portion of the silicon wafer with coherent lighthaving multiple wavelengths with a bandwidth b about a centralwavelength w_(c). The limits of the bandwidth b are defined such that awavelength for which an optical absorption coefficient of thehighly-doped layer of the silicon wafer is a minimum lies within thebandwidth b. Moreover, the method comprises the step of determining athickness of the silicon wafer by analyzing the radiation reflected bythe silicon wafer using an optical coherence tomography process. Theoptical coherence tomography process may be one of those processesdisclosed above.

This method according to the invention provides the advantages alreadymentioned in connection with the apparatus for monitoring which are notmentioned again in order to avoid repetition.

In an embodiment, the bandwidth b has limits that are defined such thatthe central wavelength w_(c) is the wavelength for which an opticalabsorption coefficient of the highly-doped layer of the silicon wafer isa minimum.

The method may further comprise the steps of providing an apparatus forthinning a silicon wafer according to one of the embodiments mentionedabove, attaching the silicon wafer to the apparatus for thinning,removing at least a portion of the backside of the silicon wafer, andstopping the removing step, wherein the steps of illuminating, receivingand determining are performed at least during the step of removing.

The method may further comprise the steps of inputting a dopantconcentration of the highly-doped layer of the silicon wafer, andselecting the central wavelength w_(c) to be the wavelength for whichthe optical absorption coefficient of the highly-doped layer of thesilicon wafer having the selected dopant concentration is minimal.

In a further embodiment, the apparatus for thinning comprises a storagedevice with stored data of a dependence of the absorption coefficient ofsilicon on the dopant concentration and the central wavelength w_(c) isselected using the data stored in the storage device.

The coherent light may be emitted from a spectral broadband light sourcewhich may be selected from the group consisting of a light-emittingdiode, a semiconductor superluminescent diode (SLD) and an opticallypumped fiber based amplified spontaneous emission (ASE) source, or froma tunable laser source which may be selected from the group consistingof an optically pumped photonic crystal laser and a semiconductorquantum dot tunable laser.

The central wavelength w_(c) fulfills preferably the relation 950nm≦w_(c)≦1150 nm. Moreover, the highly-doped layer of the silicon wafercomprises preferably silicon with a dopant concentration N, wherein1*10¹⁹ cm⁻³≦N≦1*10²¹ cm⁻³.

In a further embodiment, a first predetermined thickness for thethickness of the silicon wafer is set and the removing step is stoppedwhen the determined thickness of the silicon wafer reaches the firstpredetermined thickness.

The silicon wafer may have an active device region and a reinforcing ribregion, the active device region having plural devices formed on a frontside of the silicon wafer, the front side being opposite to the backsideof the silicon wafer, wherein the reinforcing rib region is arranged atan outer circumferential edge of the silicon wafer. In this embodiment,the method further comprises the steps of setting a second predeterminedthickness for the thickness of the active device region, wherein thesecond predetermined thickness is smaller than the first predeterminedthickness, removing at least a portion of the backside of the activedevice region after the removing step such that the reinforcing ribregion is formed to be thicker than an inside region of the siliconwafer comprising the active device region, illuminating at least aportion of the active device region with coherent light, and receivingat least a portion of radiation reflected by the active device region.The method further comprises the steps of determining a thickness of theactive device region by analyzing the radiation reflected by the activedevice region using an optical coherence tomography process which may beselected from the group consisting of a 1D-se FD OCT process, a 1D-te FDOCT process, a 1D-se TD OCT process and a 1D-te TD OCT process, andstopping the second removing step when the determined thickness of theactive device region reaches the second predetermined thickness. In thisembodiment, strength is provided to the semiconductor waver by forming athick rib on the outer circumferential edge of the silicon wafer. Thismay aid in preventing the breakage of the active device regioncontaining the semiconductor devices.

The first predetermined thickness and/or the second predeterminedthickness may be set in the coupling unit. Preferably, the firstpredetermined thickness and/or the second predetermined thickness areset by means of a touch screen of the coupling unit.

In a further embodiment which relates to the 1D-se FD OCT process or the1D-te FD OCT process, the spectral distribution of the radiationreflected by the silicon wafer and/or by the active device region isexpanded using a diffraction grating.

In a yet further embodiment, partial intensities I(λ) of the radiationreflected by the silicon wafer and/or by the active device region aremeasured as a function of the wavelengths λ with an array ofphotodetectors in the spectrometer and a spectrum I′(1/λ) is calculatedfrom the measured partial intensities I(λ) as a function of the invertedwavelengths.

In a further embodiment, the rotation speed of the rotatable plateand/or the rotatable grinding wheel is adjusted depending on thedetermined thickness of the silicon wafer and/or the determinedthickness of the active device region.

In a further embodiment, the steps of removing, illuminating, receivingand determining are performed at each of a plurality of angularpositions during a rotation of the silicon wafer, thereby determining aplurality of determined thicknesses for said rotation of the siliconwafer. In this embodiment, the method further comprises the steps ofcomparing at least two of the plurality of determined thicknesses, anddetermining if a difference between the at least two of the plurality ofdetermined thicknesses surpasses a predetermined difference. Thisembodiment may be used to determine if wedging of the silicon waferoccurs during grinding and, if wedging occurs, to provide levelinginformation to the thinning apparatus, such that a grinding surface ofthe grinding wheel and/or a surface of the plate can be adjusted toreduce or eliminate wedging.

The invention further relates to the use of the apparatus for monitoringaccording to one of the mentioned embodiments for monitoring a thicknessof a silicon wafer with a highly-doped silicon layer at least at abackside of the silicon wafer during a backside grinding process of thesilicon wafer, wherein the highly-doped layer of the silicon wafercomprises silicon with a dopant concentration N, wherein 1*10¹⁹cm⁻³≦N≦1*10²¹ cm⁻³.

The abstract of the disclosure is provided for the sole reason ofcomplying with the rules requiring an abstract, which will allow asearcher to quickly ascertain the subject matter of the technicaldisclosure of any patent issued from this disclosure. It is submittedwith the understanding that it will not be used to interpret or limitthe scope or meaning of the claims.

Any advantages and benefits described may not apply to all embodimentsof the invention. When the word “means” is recited in a claim element,Applicant intends for the claim element to fall under 35 USC 112,paragraph 6. Often a label of one or more words precedes the word“means”. The word or words preceding the word “means” is a labelintended to ease referencing of claims elements and is not intended toconvey a structural limitation. Such means-plus-function claims areintended to cover not only the structures described herein forperforming the function and their structural equivalents, but alsoequivalent structures. For example, although a nail and a screw havedifferent structures, they are equivalent structures since they bothperform the function of fastening. Claims that do not use the word meansare not intended to fall under 35 USC 112, paragraph 6.

What is claimed is:
 1. An apparatus for monitoring a thickness of asilicon wafer with a highly-doped layer at least at a backside of thesilicon wafer, the apparatus comprising: a light source configured toemit coherent light of multiple wavelengths, a measuring head configuredto be contactlessly positioned adjacent the silicon wafer comprising thebackside from which a portion is to be removed and configured toilluminate at least a portion of the silicon wafer with the coherentlight of multiple wavelengths and configured to receive at least aportion of radiation reflected by the silicon wafer, a spectrometerconfigured to receive at least a portion of the radiation reflected bythe silicon wafer and measure the partial intensities of the radiationreflected by the silicon wafer as a function of wavelength, a beamsplitter coupled to the measuring head, the light source and thespectrometer, a selection device configured to select the centralwavelength W_(c) to be a wavelength for which an optical absorptioncoefficient of the highly-doped layer of the silicon wafer having theinput dopant concentration is a minimum, an evaluation device configuredto determine a thickness of the silicon wafer by analyzing the radiationreflected by the silicon wafer using an optical coherence tomographyprocess, wherein the light source is configured to emit coherent lightat multiple wavelengths with a bandwidth b around a central wavelengthW_(c), the bandwidth b having limits that are defined such that awavelength for which an optical absorption coefficient of thehighly-doped layer of the silicon wafer is a minimum lies within thebandwidth b, wherein the apparatus is an in-situ backside grindingprocess apparatus for monitoring a thickness of the silicon wafer duringa backside grinding process and wherein the evaluation device isconfigured to determine the thickness of the silicon wafer by analyzingthe radiation reflected by the silicon wafer during the backsidegrinding process, and further comprising an input device for inputting adopant concentration of the highly-doped layer of the silicon wafer andwherein the apparatus is configured to select the central wavelengthW_(c) to be the wavelength for which the optical absorption coefficientof the highly-doped layer of the silicon wafer having the input dopantconcentration is a minimum.
 2. The apparatus of claim 1, wherein thebandwidth b has limits that are defined such that the central wavelengthw_(c) is the wavelength for which an optical absorption coefficient ofthe highly-doped layer of the silicon wafer is a minimum.
 3. Theapparatus of claim 1, further comprising a storage device with storeddata of a dependence of the absorption coefficient on the dopantconcentration and wherein the apparatus is configured to select thecentral wavelength w_(c) using the data stored in the storage device. 4.The apparatus of claim 1, wherein w_(c) is in the range of 950nm≦w_(c)≦1150 nm and the highly-doped layer of the silicon wafercomprises silicon with a dopant concentration N, wherein N is in therange of 1*10¹⁹ cm⁻³≦N≦1*10²¹ cm⁻³.
 5. The apparatus of claim 4, whereinthe spectral broadband light source is selected from the groupconsisting of a light-emitting diode, a semiconductor superluminescentdiode (SLD) and an optically pumped fiber based amplified spontaneousemission (ASE) source, or wherein the tunable laser source is selectedfrom the group consisting of an optically pumped photonic crystal laserand a semiconductor quantum dot tunable laser.
 6. The apparatus of claim1, wherein the light source is a spectral broadband light source or atunable laser source.
 7. The apparatus of claim 1, wherein the opticalcoherence tomography process is selected from the group consisting of a1D-se FD OCT process (one dimensional spatially encoded Fourier DomainOptical Coherence Tomography process), a 1D-te FD OCT process (onedimensional time encoded Fourier Domain Optical Coherence Tomographyprocess).
 8. The apparatus of claim 1, wherein the optical coherencetomography process is a 1D-te TD OCT process (one dimensional timeencoded Time Domain Optical Coherence Tomography process).
 9. Theapparatus of claim 1, wherein the optical coherence tomography processis a 1D-se TD OCT process (one dimensional spatially encoded Time DomainOptical Coherence Tomography process).
 10. The apparatus of claim 1,wherein the beam splitter is an optical coupler.
 11. The apparatus ofclaim 10, further comprising: at least one first optical waveguide, theat least one first optical waveguide coupling the measuring head to theoptical coupler, at least one second optical waveguide, the at least onesecond optical waveguide coupling the optical coupler to the lightsource, at least one third optical waveguide, the at least one thirdoptical waveguide coupling the optical coupler to the spectrometer. 12.The apparatus of claim 1, wherein the spectrometer comprises adiffraction grating configured to expand the spectral distribution ofthe radiation reflected by the silicon wafer.
 13. The apparatus of claim1, further comprising a housing, wherein at least the measuring head isarranged in the housing and wherein the housing comprises a window beingtransparent for the light of the light source.
 14. A system for thinninga portion of a highly-doped side of a silicon wafer, the systemcomprising: a thinning apparatus for removing a portion of the siliconwafer, the thinning system comprising: a rotatable grinding wheel, agrinding motor coupled to the grinding wheel and configured forselectively rotating the grinding wheel about a first rotational axis, arotatable plate configured to retain the silicon wafer, a plate motorcoupled to the rotatable plate, a control unit configured to halt thegrinding wheel motor and the plate motor when silicon wafer reaches apredetermined thickness, a monitoring apparatus in communication withthe control unit, the monitoring apparatus comprising a light sourceconfigured to emit coherent light of multiple wavelengths, a measuringhead configured to be contactlessly positioned adjacent the siliconwafer comprising the backside from which a portion is to be removed andconfigured to illuminate at least a portion of the silicon wafer withthe coherent light of multiple wavelengths and configured to receive atleast a portion of radiation reflected by the silicon wafer, aspectrometer configured to receive at least a portion of the radiationreflected by the silicon wafer and measure the partial intensities ofthe radiation reflected by the silicon wafer as a function ofwavelength, a beam splitter coupled to the measuring head, the lightsource and the spectrometer, a selection device configured to select thecentral wavelength W_(c) to be a wavelength for which an opticalabsorption coefficient of the highly-doped layer of the silicon waferhaving the input dopant concentration is a minimum, an evaluation deviceconfigured to determine a thickness of the silicon wafer by analyzingthe radiation reflected by the silicon wafer using an optical coherencetomography process, wherein the light source is configured to emitcoherent light at multiple wavelengths with a bandwidth b around acentral wavelength W_(c), the bandwidth b having limits that are definedsuch that a wavelength for which an optical absorption coefficient ofthe highly-doped layer of the silicon wafer is a minimum lies within thebandwidth b, and an input device for inputting a dopant concentration ofthe highly-doped layer of the silicon wafer and wherein the monitoringapparatus is configured to select the central wavelength W_(c) to be thewavelength for which the optical absorption coefficient of thehighly-doped layer of the silicon wafer having the input dopantconcentration is a minimum.
 15. The apparatus according to claim 14,further comprising a coupling unit, wherein the coupling unit is coupledto the evaluation device and to the control unit and wherein the controlunit is configured to halt the system for removing when the determinedthickness of the silicon wafer reaches a predetermined thickness. 16.The apparatus of claim 15, wherein the coupling unit is a part of the insitu backside grinding process apparatus.
 17. The apparatus of claim 15,wherein the coupling unit is a part of the control unit.
 18. Theapparatus of claim 15, wherein the control unit comprises a controllerconfigured to adjust the rotation speed of the rotatable plate and/orthe rotatable grinding wheel depending on the determined thickness ofthe silicon wafer.
 19. A method for monitoring and adjusting a thicknessof a silicon wafer with a highly-doped layer at least at a backside ofthe silicon wafer, the method comprising: illuminating at least aportion of the silicon wafer with coherent light comprising multiplewavelengths with a bandwidth b around a central wavelength w_(c), thebandwidth b having limits that are defined such that a wavelength forwhich an optical absorption coefficient of the highly-doped layer of thesilicon wafer is a minimum lies within the bandwidth b, receiving atleast a portion of radiation reflected by the silicon wafer, anddetermining a thickness of the silicon wafer by analyzing the radiationreflected by the silicon wafer using an optical coherence tomographyprocess whilst removing a portion of the backside of the silicon wafer,and inputting a dopant concentration of the highly-doped layer of thesilicon wafer, and selecting the central wavelength w_(c) to be thewavelength for which the optical absorption coefficient of thehighly-doped layer of the silicon wafer having the input dopantconcentration is minimal.
 20. The method of claim 19, wherein thebandwidth b has limits that are defined such that the central wavelengthw_(c) is the wavelength for which an optical absorption coefficient ofthe highly-doped layer of the silicon wafer is a minimum.
 21. The methodof claim 19, further comprising: providing an apparatus for thinning asilicon wafer, attaching the silicon wafer to the apparatus forthinning, removing at least a portion of the backside of the siliconwafer, and stopping the removing step, wherein the steps ofilluminating, receiving and determining are performed at least duringthe step of removing.
 22. The method of claim 19, wherein the siliconwafer is illuminated with coherent light emitted from a spectralbroadband light source or a tunable laser source.
 23. The method ofclaim 19, wherein the thickness of the silicon wafer is determined usingthe optical coherence tomography process selected from the groupconsisting of a 1D-se FD OCT process (one dimensional spatially encodedFourier Domain Optical Coherence Tomography process), a 1D-te FD OCTprocess (one dimensional time encoded Fourier Domain Optical CoherenceTomography process).
 24. The method of claim 19, wherein the thicknessof the silicon wafer is determined using the optical coherencetomography process is a 1D-te TD OCT process (one dimensional timeencoded Time Domain Optical Coherence Tomography process).
 25. Themethod of claim 19, wherein the thickness of the silicon wafer isdetermined using the optical coherence tomography process is a 1D-se TDOCT process (one dimensional spatially encoded Time Domain OpticalCoherence Tomography process).
 26. The method of claim 19, wherein theapparatus for thinning comprises a storage device with stored data of adependence of the absorption coefficient on the dopant concentration andwherein the central wavelength w_(c) is selected using the data storedin the storage device.
 27. The method of claim 19, wherein the portionof the silicon wafer is illuminated using a light-emitting diode, asemiconductor superluminescent diode (SLD), an optically pumped fiberbased amplified spontaneous emission (ASE) source, an optically pumpedphotonic crystal laser or a semiconductor quantum dot tunable laser. 28.The method of claim 19, wherein 950 nm≦w_(c)≦1150 nm.
 29. The method ofclaim 19, wherein the highly-doped layer of the silicon wafer comprisessilicon with a dopant concentration N, wherein 1*10¹⁹ cm⁻³≦N≦110²¹ cm⁻³.30. The method of claim 19, wherein a first predetermined thickness forthe thickness of the silicon wafer is set and wherein the removing stepis stopped when the determined thickness of the silicon wafer reachesthe first predetermined thickness.
 31. The method of claim 30, whereinthe first predetermined thickness and/or the second predeterminedthickness are set in the coupling unit.
 32. The method of claim 31,wherein the first predetermined thickness and/or the secondpredetermined thickness are set by means of a touch screen of thecoupling unit.
 33. The method of claim 19, wherein the silicon wafer hasan active device region and a reinforcing rib region, the active deviceregion having plural devices formed on a front side of the siliconwafer, the front side being opposite to the backside of the siliconwafer, and wherein the reinforcing rib region is arranged at an outercircumferential edge of the silicon wafer and wherein the method furthercomprises the steps of: setting a second predetermined thickness for thethickness of the active device region, removing at least a portion ofthe backside of the active device region, illuminating at least aportion of the active device region with the coherent light, receivingat least a portion of radiation reflected by the active device region,determining a thickness of the active device region by analyzing theradiation reflected by the active device region using an opticalcoherence tomography process, stopping the second removing step when thedetermined thickness of the active device region reaches the secondpredetermined thickness.
 34. The method of claim 19, wherein thespectral distribution of the radiation reflected by the silicon wafer isexpanded using a diffraction grating.
 35. The method of claim 19,wherein partial intensities I(λ) of the radiation reflected by thesilicon wafer are measured as a function of the wavelengths λ with anarray of photodetectors in the spectrometer and a spectrum I′(1/λ) iscalculated from the measured partial intensities I(λ) as a function ofthe inverted wavelengths.
 36. The method of claim 19, wherein a rotationspeed of a rotatable plate and/or a rotatable grinding wheel areadjusted depending on the determined thickness of the silicon wafer. 37.The method of claim 19, further comprising the steps of: performing thesteps of removing, illuminating, receiving and determining at each of aplurality of angular positions during a rotation of the silicon wafer,thereby determining a plurality of determined thicknesses for saidrotation of the silicon wafer, comparing at least two of the pluralityof determined thicknesses, and determining if a difference between theat least two of the plurality of determined thicknesses surpasses apredetermined difference.